The present invention relates to a process for the production of a multi-layer lacquer coating by dry-on-wet application of a powder coating composition onto a substantially uncrosslinked, previously electrophoretically deposited lacquer layer, followed by the joint baking of these lacquer layers.
Industrial lacquer coating is distinguished by efforts to optimise lacquering processes in terms of environmental friendliness and energy consumption. Ways of moving towards this objective are, for example, the use of powder coating systems and economising on energy-intensive processing stages, such as for example reducing the number of baking stages. It is customary in this connection, in order not to expose the lower lacquer layers to excessive temperatures, when baking the individual layers for the baking temperatures of subsequent layers to be less than those of the preceding layers.
The application of a powder coating composition onto a dried, but uncrosslinked, previously electrophoretically deposited lacquer layer is known from JP 62 238 398 and from JP 63 274 800.
It is explained in EP-B-0 240 565 that powder coatings based on solid aromatic epoxy resins with an average of less than two epoxy groups per molecule have inadequate resistance to exposure to stress to ASTM D 2794 (impact indentation, impact test). Epoxy resin based powder coatings with an epoxy functionality of greater than 2 are thus used. The possibility of dry-on-wet application is not described.
EP-A-0 292 771 describes coating compositions which provide protection against stone impact based on epoxy resins which are elasticised by chemical modification with diene polymers. The coating compositions may be in powder form and may be applied to a crosslinked or uncrosslinked electrocoated lacquer layer.
EP-A-0 440 292 explains .that stability and viscosity problems occur in such powder coatings containing such elastomer-modified epoxy resins as described in EP-A-0 292 771. EP-A-0 440 292 thus describes the formulation of epoxy resin based powder coatings which contain elastomer-modified phenolic hardeners. These powder coatings may also be applied to a crosslinked or uncrosslinked electrocoated lacquer layer, wherein multi-layer lacquer coatings with good resistance to exposure to stress to ASTM D 2794 and good stone impact resistance are obtained.
It has been found that coating layers formed from the above-described powder coatings have a tendency to yellowing and embrittlement.
EP-A-0 449 359 moreover describes overcoming the stability problems of the elastomer-modified epoxy resins of EP-A-0 292 771 by producing a differently elastomer-modified epoxy resin. To this end, a carboxy-functional hydrogenated diene/vinyl aromatic block copolymer is dispersed in the liquid epoxy resin and reacted with it with the addition of catalyst and polyphenol. The powder coatings formulated on the basis of the epoxy resin elastomer-modified in this manner may be applied onto crosslinked or uncrosslinked electrocoated lacquer layers to yield multi-layer lacquer coatings resistant to stone impact.
All these powder coatings with binders or hardeners elastomer-modified in this manner, in particular the powder coating described in EP-A-0 449 359, have the common feature that the elastomer-modified components require elaborate synthesis.